The Moscow Times / (Bloomberg) - The U.S. and Russia clashed Tuesday in the United Nations Security Council over the latest fighting in Syria, as the secretary-general's special envoy said this was the most "violent, worrying and dangerous" moment in the past four years.

All parties must "de-escalate immediately and unconditionally," Staffan de Mistura told the Security Council, urging Russia, Iran and Turkey to use their influence to reduce violence. Renewed bombing by forces of Syrian leader Bashar Al-Assad had killed 1,000 civilians in the first week of February.

"We are on the brink of dangerous and worrying escalation of violence," de Mistura said. "Civilians have been killed on a horrific scale. There have been several allegations of chemical attacks. If confirmed, it is outrageous."

Read More Putin's Goals in Syria Went Beyond Saving Assad Nikki Haley, the U.S. Ambassador to the UN, blamed Iran for the latest round of fighting and said advisers from the Islamic Republic and its ally, the Lebanese Hezbollah, planned on staying in Syria.

"We can't offer support for peace on the one hand and ignore the fact that the chief sponsor of terrorism in the Middle East and its terrorist militia are digging in," Haley told the council, also blaming Russia for failing to control the violence. "Russia was also supposed to guarantee that all chemical weapons would be removed from Syria. Instead, we see the Assad regime continue to bomb, starve, and yes, gas civilians."

De Mistura said escalating violence between different parties undermined prospects for a political solution to the civil war, which next month will enter its eight year. The Syrian government has not engaged in the UN-led talks in Vienna or Geneva, de Mistura said. More than 340,000 people have been killed in the fighting and millions have been displaced.

Read More Russia Dismisses Reports of Syria Casualties as 'Classic Disinformation' "Something is being constantly demanded of the Russian federation," Russia's ambassador Vassily Nebenzia said. He said the U.S. should use its influence with its allies to reduce violence rather then encouraging them to boycott Russian-sponsored peace efforts. "There are forces that wish to torpedo the political process."

Russia's intervention in Syria in September 2015, along with Iranian and Hezbollah forces, succeeded in turning the tide in Assad's favor.

Other council members also expressed concern over the escalation in the fighting.

"All the ingredients are present if we don't do anything about it urgently for a major regional and international confrontation," said France's ambassador Francois Delattre. "The potential for an enlargement of the conflict should be taken very seriously."

INGLES: Russia Clashes With U.S. at UN Over Escalating Violence in Syria